1. Field of the Invention
The present invention relates to apparatus for measuring a running condition of a vehicle by detecting rotation of a wheel of the vehicle.
2. Related Background Art
Wheeled vehicles running on a road or rails include a bicycle, automobile, electric car or train etc. Computer-based running condition measuring apparatus are known which measure or compute a running condition of the vehicle (e.g., vehicle speed, traveled distance). Particularly, when applied to the bicycle, such a running condition measuring apparatus is called "cycle computer."
For example, in the cycle computer, a magnet is mounted on one of spokes of a front wheel while a magnetic detector is fixed to a fork blade to detect passing of the magnet.
In the prior art, a reed switch has been used as the magnetic detector. The reed switch is normally kept in an OFF condition. When the magnet passes by the reed switch as the wheel rotates, the reed switch is turned ON. Thus it generates a magnet detection signal (reed switch signal) every revolution of the wheel. The number of revolutions of the wheel per unit time is measured from the detection signal. The cycle computer CPU computes a traveled distance and/or a speed of the bicycle by multiplying the detected number of revolutions of the wheel by wheel circumference data previously stored. A display device such as a liquid crystal display device (LCD) displays the computed results.
In the prior art running measuring apparatus, continuous power supply is provided to various components of the apparatus (e.g., display device) even when the vehicle is stopped or at rest. This results in wasteful power consumption.
Some prior art cycle computer employs a transmitter (first device) including a rotation detector, and a receiver (second device) including a CPU for computing the running condition. The first and second devices are independently powered by their own battery. When the battery power in the receiver is used up, the transmitter can no longer send a rotation signal to the receiver. In the absence of the rotation signal, CPU in the receiver mistakes the power failure of the receiver for the stopped condition of the vehicle.